Optical properties apparatus, the restoration method, and an optical system used in the apparatus

ABSTRACT

The objectives of the present invention are to prevent or inhibit the deterioration effects such as light transmission, diffraction, reflection, spectrum generation, and interference, and these combinations, and by so doing, decrease the frequency of maintenance operations such as window replacement and to reduce the costs for such operations. This invention is characterized by steps of creating a near vacuum zone with a presence of active energy to excite an oxidation reaction of carbon wherein the near vacuum zone faces the lighting surfaces of the optical system; generating negative ions or radicals in the near vacuum zone such as unstable chemical seeds containing oxygen atoms, such as OH radicals, OH ions, ozone, O 2  ions, O-radicals; and removing or reducing the accumulated carbon which deposits on the lighting surface, by reacting the deposited carbon with the negative ions or radicals. More specifically, the method according to this invention is characterized by the step of supplying active energy while supplying a flow of gases containing oxygen atoms such as water gas or oxidizing gas (for example, water vapor, oxygen, hydrogen peroxide, ozone or mixtures of said gases with inactive gases (including air)) into the near vacuum zone, thereby removing or reducing the accumulated carbon which deposits on the lighting surface by exciting the oxidation reaction of the accumulated carbon with the supplied active energy.

BACKGROUND OF THE INVENTION

This application is a Divisional of application Ser. No. 10/833,998,filed on Apr. 29, 2004 now U.S. Pat. No. 7,190,512, the entire contentsof which are hereby incorporated by reference and for which priority isclaimed under 35 U.S.C. § 120.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for improvingthe reliability and longevity of optical properties of an optical systemby preventing, suppressing, or improving degradation of opticalproperties of an optical system lying in output light or along a lightpath of said output light, wherein said optical system is providedwithin a near vacuum zone where organic components may be decomposed,said degradation is caused by carbon deposited or accumulated upon lightirradiation surfaces, light reflection surfaces, light emission surfaces(collectively called ‘lighting surfaces’) of said optical system, andsaid surfaces faces said vacuum zone. More specifically, it relates toan optical properties restoration apparatus and a method to use it, andsuch optical properties restoration apparatus is used for improving theoptical property of a variety of optical systems provided outside of alight transmitting window of a variety of optical apparatus that producecombination effects of light transmission, refraction, reflection,spectrum generation and interference etc. by using high photon energylight, such as conventional ultraviolet light or vacuum ultravioletlight.

Furthermore, this invention relates to optical systems in a variety ofoptical apparatus and a method to use it. The optical systems improvethe optical properties inside of a light transmitting window of avariety of optical apparatus that produce combination effects of lighttransmission, refraction, reflection, spectrum generation andinterference etc. and the optical systems are provided on the light pathof high photon energy light, such as conventional ultraviolet light orvacuum ultraviolet light. More specifically, the invention can beapplied to the optical systems equipped with lenses, windows, etalons,prisms, reticles, and reflecting mirrors, etc., and high photon energylamps equipped with such optical systems. Further, the invention can beapplied to not only optical measurement equipments such asspectrometers, fluorescent light meters, interference meters,diffraction meters, but also to a variety of optical equipments thatincorporate standard light sources for vacuum ultraviolet light, lightsources for exciting chemical reactions, printing plate and photographicapplications, and various light sources for experimental applications.

DESCRIPTION OF THE RELATED ART

FIG. 14 will be used to explain the component parts and operation of amicrowave excited hydrogen ultraviolet lamp as an example of aconventional optical output apparatus to which this invention may beapplied. This apparatus is described in Non-Patent Publication 1, whichis “written by James A, R. Samson, Techniques of Vacuum UltravioletSpectroscopy, Pied Publications, Lincoln, Nebr., 1967, P159, FIG. 5.56”.The microwave oscillator 4 has a sealed tube shaped component which isprovided with both ends made of an identical electrical conductivematerial. The inner diameter and the length of the tube are determinedby the frequency of the used microwave, and the electromagnetic fielddistribution to be excited in the microwave oscillator.

Microwave oscillator tuner 18 is a tube shaped component that is anessential component of the microwave oscillator that allows theadjustment of the microwave electromagnetic field distribution of themicrowave oscillator, and its inside diameter is such that it envelopesdischarge tube 1. Further, it is inserted concentrically with endsurface of microwave oscillator 4 along their bore axes, and itsstructure is such that it may slide in the axial direction as itmaintains its role as an electrical guide for the microwave oscillator4. Like microwave oscillator 4, the material used to form tuner 18 iscopper or brass. The function of adjusting the microwave electromagneticfield distribution by said tuner 18 is performed by adjusting itsinsertion depth while generating discharge plasma 7 to thereby put themicrowave concentration 6 into the desired generation position.

Further, discharge tube 1 is installed in a manner such that it passesthrough both end surfaces of microwave oscillator 4. Although it isgenerally true that greatest electrical field is generated along thebore axis of discharge tube 1, which lies along the bore axis ofmicrowave oscillator 4, this is not always the case. The cross-sectionalshape of discharge tube 1 is round, but it could equally well be square,etc.

Discharge tube 1 functions as the vacuum boundary, the flow path for thedischarge gas, and the space in which the discharge plasma is generated.In the example illustrated in FIG. 14, in order to limit the space inwhich the discharge plasma is generated, the inside tube of theconductor has been extended along discharge tube 1 from the end surfaceof microwave oscillator 4 toward the inside of the microwave oscillator.Accordingly, discharge plasma 7 is generated in the space between theend of microwave oscillator tuner 18 and the end of the foregoing insidetube.

Microwave oscillator 4 is connected with microwave supply connector 5that delivers the microwaves. Here, the shape of the connector iscoaxial, but it could also be of the waveguide type. Either coaxialcable or coaxial pipe may be used as the transmission feed path to thecoaxial connector.

Flange 17 is attached via O-ring 13 to hold the lamp in place, on theend of discharge tube 1 at the microwave oscillator tuner 18 side. Thereis an opening at the center of the foregoing Flange 17, which has aninside diameter corresponding to that of discharge tube 1, which therebyallows the extraction of the light emitted from discharge plasma 7 inthe axial direction of discharge tube 1.

Light transmitting window 8 that is mounted in the opening of theforegoing Flange 17 serves two functions. One is as the vacuum boundaryinside of discharge tube 1 to the atmosphere. The second is to allow theextraction of the light emitted from discharge plasma 7 to outside ofthe vacuum. The foregoing microwave oscillator is described in detail inNon-Patent Publication 2 which is “.written by E. L. Ginzton, MicrowaveMeasurements, McGraw-Hill, New York, 1957”.

Discharge lamps having the above described constitution experience theproblems described below, but before describing them, definitions of theterms used will be specified.

The vacuum ultraviolet range is the wavelength range of 0.2 to 200 nm.Light inside of that range will be termed ultraviolet light or vacuumultraviolet light. The conventional wavelength for ultraviolet light is200 to 380 nm (see Dictionary of Physics, published by Baifukan, and theRika Nenpyo published by the National Observatory of Japan).

In FIG. 14, to distinguish the surfaces of the light transmittingwindow, the surface facing plasma discharge 7 shall be called innersurface 10, while the surface on the other side shall be called outsideouter surface 11.

Problems in the Degradation of Optical Properties Outside of the LightTransmitting Window

In FIG. 14, when light generated by discharge plasma 7, especiallyultraviolet light and vacuum ultraviolet light, it irradiates throughinner surface 10 of transmitting window 8, passes through transmittingwindow 8, and passes from outside outer surface 11 of light transmittingwindow 8.

Here, when ultraviolet or vacuum ultraviolet light is emitted in theatmosphere such light causes the dramatic absorption of oxygen, carbondioxide, water vapor and the like, so normally, as shown in FIG. 14,there is a mechanism on the left side of Flange 17 (to wit, outside oflight transmitting window 8) which helps maintain the vacuum. The zonein which this vacuum is maintained shall be called “vacuum zone” below.

Normally, any one of a variety of vacuum pumps may be used as themechanism to maintain the vacuum. Although there are a variety of drypumps that are oil free (which give off almost no organic gases) thatare suitable for use, rotary type pumps are the most common. Therefore,vacuum zone 14 contains organic gas from the vapor pressure of the oilused in the pump.

Further, stainless steel or aluminum metal parts, or rubber sealingparts such as O-rings are also contained within vacuum zone 14, anddepending upon the application, vacuum zone 14 may contain opticalelements such as samples, lenses, diffraction elements, mirrors,filters, transmitting windows, stages or other positioning elements,etc. Ideally, all of the materials in contact with the above describedvacuum zone 14, to wit, the stainless steel containers, aluminumcontainers, O-rings and other sealing materials, optical elements, worksamples, position adjustment mechanisms and the like, should be oil free(meaning that they should themselves give off almost no organic gases).

It is especially the case in the semiconductor industry that as theprocessing size (the width of the circuit lines) becomes finer andfiner, the wavelength of the light used to make the exposure pattern forthe circuit has reached the vacuum ultraviolet light range. For examplethe wavelength of the fluorinated argon excimer laser used as the lightsource for such applications is 193 nm (which when converted to energyis 6.4 eV), but in recent years, development has proceeded on laserstepper apparatus that generate wavelengths of 157 nm.

However, in actual practice, it is difficult to avoid the emission oforganic gases inside vacuum zone 14, which can arise from variousfactors such as lubricating oil that is used for the mechanical drivestructure, contamination of the work sample, out gassing from theO-rings, out gassing from plastic parts, inadequate degreasing orcleaning of the parts, or contamination introduced by human error. Thus,in actual applications, the presence of organic gases inside theforegoing vacuum zone must be considered.

Organic gases inside vacuum zone 14 have a certain probability of beingadsorbed onto outside outer surface 11 of light transmitting window 8.This adsorption probability varies according to the material comprisingthe light transmitting window 8 and the type of organic gases, but theappearance of the adsorption phenomena itself is unavoidable.

When organic gases are adsorbed onto outer surface 11, at the same time,ultraviolet light, especially vacuum ultraviolet light, generated by theplasma irradiates these organic gases, which causes the directexcitation of the organic gas molecules to put them in an active state.This produces reactions, which draw the hydrogen, a component element ofthe organic gases in a dehydrogenation reaction, and finally, theadsorbed organic gas is converted into carbon (graphite). When thisstate is reached, it is no longer a gas but a solid, which affixesitself to and accumulates on outer surface 11 of light transmittingwindow 8. The carbon accumulation then adsorbs new organic gases, andtheir irradiation by ultraviolet light, especially vacuum ultravioletlight, converts them to carbon as well, which causes the buildup toproceed. As this process continues, outer surface 11 of lighttransmitting window 8 becomes covered by a carbon film. Since carbon isblack, it absorbs light of various wavelengths, and as the accumulationof the carbon on outer surface 11 continues, the transmission ratethrough light transmitting window 8 gradually diminishes.

Here, for simplicity of explanation, it was assumed that the organicgases are hydrocarbon gases and that a dehydrogenation reaction resultedin their conversion to graphite, but in actual practice, the organicgases may include other-than-hydrocarbon elements such as oxygen,nitrogen, iodine, fluorine, chlorine, etc., and such organic gases canbe adsorbed onto outer surface 11 of light transmitting window 8 just ascan hydrocarbon gases, and then, through the action of ultravioletlight, especially vacuum ultraviolet light, are converted and nongaseouscomponents are left as residuals. Thus, strictly speaking the buildup isnot graphite, but it is an amorphous solid having carbon as its primarycomponent. For purposes of describing this invention, this solidprimarily comprised of carbon shall be termed “carbon.”

The phenomenon of carbon buildup requires the adsorption of organicgases and their irradiation by ultraviolet light, especially vacuumultraviolet light. As the accumulation of carbon proceeds, the intensityof the ultraviolet light, especially vacuum ultraviolet light, that isemitted from outer surface 11 where carbon has accumulated issignificantly diminished. Carbon buildup will continue until all of thelight intensity is sapped. At that time, new dehydrogenation reactionscannot take place and the accumulation of the carbon film stops.Accordingly, this process is not one where a carbon film can growwithout limits, but the phenomenon ceases once a limit film thicknesshas been reached.

Normally, the phenomenon of carbon buildup on outer surface 11 of theforegoing light transmitting window 8 does not proceed rapidly. Theproblem is one of diminishing transmission through the lighttransmitting window 8 over a long period of time. In spectrographicapplications, when the quantity of light from the light sourcediminishes, it creates drift which affects the accuracy of themeasurements, and in applications involving surface treatments byultraviolet light irradiation, problems arise due to inadequateprocessing caused by diminished irradiation intensity.

One means of addressing this problem of the carbon buildup phenomenon isto strive for an oil-free vacuum zone 14, however, once an organicsubstance has contaminated vacuum zone 14, the cleanup process isextremely difficult. Accordingly, the conventional countermeasure todiminished transmission rates due to carbon accumulations on outersurface 11 of light transmitting window 8 involve using cleansers orpolishing to remove the carbon to restore light transmitting window 8 toits original state, or replacing light transmitting window 8 entirely.

In the prior art, the decline in the light transmission rate of lighttransmitting window 8, to wit, its degradation, was the determiningfactor in lamp longevity. Lamps that had reached their longevity, hadtheir light transmitting windows 8 cleaned or replaced, which requiredbreaking the vacuum in vacuum zone 14 or in the lamp. This operationrequired several hours time during which the lamp could not be used.

Next, conventional countermeasures in response to degradation of thetransmitting window due to carbon buildup will be described.

The technology disclosed in Japan Patent Application Publication No.2001-319618 (Patent Publication 1) will be described below.

In this example, the light source in question was a hydrogen lamp. Whenthe hydrogen is introduced into the discharge tube, a halogen is alsosealed within as a means to increase the lamp's longevity. The halogensealed therein is in the form of an organic halogen compound. This meansthat an organic halogen substance has been introduced into the dischargearea. Then, when the lamp is in operation, the organic materialdecomposes and causes a film of organic material, primarily carbon, toadhere onto the inner wall of the discharge tube. This inner wallfunctions as the light transmitting window, and the adhesion of materialon its walls invites a reduction in the quantity of light generated. Asa countermeasure, the above-cited Patent Publication 1 proposed apre-shipment treatment of the lamps that could forcibly cause a carbonfilm to adhere to the region that functioned as the light transmittingwindow where carbon was envisioned as building up during lamp operation,and then, during the normal operation of the lamp no additional carbonbuildup would occur. This technology considered that there were finitelimits to the generation of organic material and that thiscountermeasure would effectively create an environment where no newbuildup would occur during the operation of the lamp.

However, as described above for vacuum zone 14, if the apparatusinvolved was one that required repeatedly opening to the atmosphere orvacuum release (such as in a spectrographic application where sampleshave to be replaced, in applications where optic elements have to beadjusted, or where work pieces need to be exchanged during surfacetreatments, etc.), even if the spec calls for no organic contaminationto be introduced during the assembly and adjustment processes, notintroducing such contamination is rare in actual practice and hence, itwould be impossible to avoid degradation of light transmitting window 8

Further Japan Patent Application Publication No. 2001-293442 (PatentPublication 2) relates to a cleansing method to remove adsorbed organicmaterials from the surfaces of optical elements by means of a methodthat minimally includes: (1) a process to cleanse the optical elementswith an organic solvent, (2) a process to irradiate the optical elementswith ultraviolet light in the presence of oxygen, and (3) a process toheat and cleanse said optical elements. Not only does this disclosurehave an objective that differs from removing accumulated carbon filmsfrom surfaces, but it further does not resolve the problem of cleaningoptical elements such as the light transmitting window when there is aneed to break the vacuum.

Further, Japan Patent Application Publication No. 2002-219429 (PatentPublication 3) discloses technology that is similar to that of thepresent invention. Its objective is to improve the treatment precisionand treatment efficiency of cleansing, etc., and is characterized inthat the surfaces of substrates including glass substrates, syntheticresin substrates, ceramic substrates, metal substrates, and compositesubstrates comprised of 1 or a plurality of the foregoing substrates arewetted on the surface inside of a heated gas atmosphere containing watervapor and subsequently, the substrate is irradiated with ultravioletlight in a mixed atmosphere of heated inactive gas and water vapor,which is at a lower concentration than was present in the wettingatmosphere, which thereby serves to dissolve organic substances adheringto the surface of said substrate, and moreover, reduction active seeds[H—] and oxidizing active seeds [—OH] are generated, and these activeseeds [H—] and active seeds [—OH] react with the products ofdecomposition of the organic material.

The objective of said prior technology is not to remove the carbon filmfrom surfaces, but rather, it aims to dissolve the organic adherents tosubstrate surfaces by reducing them to smaller molecules, especiallywith regard to irradiating the substrate surface with ultraviolet light,and cleansing and etching processes, which use the ultraviolet lightirradiation as a means of substrate treatment. Not only does theobjective differ from the present invention, but since the substratesurface must be in a saturated condition, it is premised on the waterbeing a liquid under the reaction conditions. As a result, the methodcan only be used in an environment of near normal atmosphericpressure—it cannot perform optical element cleansing of lighttransmitting windows and the like under vacuum conditions, and it doesnothing to resolve the problems that develop when it is necessary tobreak the vacuum.

Although the explanation up to this point has been limited to thephenomenon that occurs on the outer surface 11 of light transmittingwindow 8, this type of carbon buildup phenomenon is not confined to onlyouter surface 11 of light transmitting window 8. It is generally thecase that the phenomenon of carbon buildup occurs on the surfaces ofobjects located in vacuum zone 14 that are irradiated by ultravioletlight, especially vacuum ultraviolet light, that is emitted from thelight transmitting window 8. This phenomenon is unavoidable so long asthe conditions of the presence of organic gases and ultraviolet light,especially vacuum ultraviolet light, coexist. The “objects” referred toin the foregoing explanation include the mirrors that switch the lightpath in spectroscopic applications, filters, lenses for focusing lightand diffraction elements used in spectroscopic applications, lenses usedfor focusing light, and various filters used in surface treatmentapplications, in other words, any of a variety of optical elements.Hereinafter, any of such objects will be referred to collectively as“optical elements.” When carbon accumulates on these optical elements itcauses serious problems by reducing their light transmission and lightreflectivity. In actual practice, it lowers or causes the total loss offunction of apparatus used in vacuum zone 14.

Formerly, to counter such diminishment of the light transmission andreflection, such optical elements had to be replaced with new ones, butthis approach leads to high maintenance costs and keeps the apparatusout of service for the time required for maintenance.

The problem with lamp longevity due to the deterioration of lighttransmitting window 8 is not confined to the microwave-excited hydrogenultraviolet lamps that were described in the foregoing example, similarproblems exist for a wide variety of lamps such as those using He, Ne,Ar, Kr, Xe, O2, N2, D2 (deuterium molecules), Hg, etc.; lamps using highfrequency discharge, arc discharge, glow discharge, inductive barrierdischarge, or flash discharge in their discharge mode; or in halogenlamps or carbon lamps that heat a filament using electrical current astheir means of light generation.

Problems in the Degradation of Optical Properties Inside of the OpticalSystems

The problem of lowing the light transmitting property from shortwavelength ultraviolet light is not limited to the degradation ofoptical properties outside of the light transmitting window, but alsothe degradation of optical properties inside of the light transmittingwindow.

In recent years, in order to obtain better light transmitting propertiesfrom short wavelength ultraviolet light, SiO2 has been developed for useas the foregoing light transmitting windows.

Further, a problem with mercury vapor lamps is more serious comparing tothe previous time, that the quartz glass used in them loses its clarity.The quartz glass in a mercury vapor lamp functions as the vacuumboundary to the outside for the inside of the lamp, and it alsofunctions to transmit the ultra violet light that is generated from theluminescence of the mercury, but the phenomenon losing the claritydegrades its light transmission properties and is a factor in thedetermination of lamp longevity.

In Japanese Patent Application Publication No. Hei 5-325893 (PatentPublication 4), for example, a countermeasure for losing the clarity wasproposed as using a light emitting tube as the metal vapor electricaldischarge arc tube, which employed a roughened inside surface of theglass bulb exhibiting a surface grain size of under 1 micron. So doingwould impede the crystallization (losing the clarity) of the arc tubeeven after it had been operating for long periods of time, and therebyimpede the decline in light flux (illumination sustenance rate) to makeit possible to maintain bright pictures and high quality displays over along period of time for projection type displays.

This technology was applied to quartz glass or high silicate glass usedin arc tubes, and although it would be possible to apply it toconventional ultraviolet light applications in the 250-360 nm wavelengthrange, the transmission rate of quartz glass for vacuum ultravioletlight of a wavelength of 190 nm diminishes substantially.

Further, Japanese Patent Application Publication No. Hei 3-77258 (PatentPublication 6) discloses technology for 254 nm ultraviolet lightconstant pressure mercury vapor lamps, wherein the inside surface ofsynthetic quartz glass is coated with a 1 to 3% solution of metal oxideparticles having an average particle diameter under 100 μm, which in theexamples consisted of metal oxide having an average particle diameter of20 μm.

Further, Japanese Patent Application Publication No. Hei 8-212976(Patent Publication 7) discloses technology for a discharge lamp usingan arc tube comprised of a quartz glass tube with mercury sealed withinand electrodes sealed on each end, that employed a thin film coating ofAl2O3, etc. on the inside of the tube, wherein the thin film is thickeron the inside surface of the foregoing arc tube near the center of thetube than it is in the other areas; specifically, the thick film area onthe inside surface of the foregoing arc tube is ⅓ to ½ the length of theeffective light emission length, which is the distance between theelectrodes on either end of the arc tube, and is such that the filmthickness in the aforementioned thick film area ranges from 0.2 μm to0.3 μm, while the film thickness in the other areas ranges from 0.1 μmto 0.15 μm.

However, this prior art technology related to quartz glass, especiallyto that used in low pressure mercury discharge lamps, and it regulatedthe thickness of the protective film where the mercury atoms existed asa means to deal with the problem of the mercury being deposited upon theinside wall of the arc tube to thereby lower the transmission rate oflight through the quartz glass and cause the blackening of the dischargelamp, which further diminishes its irradiation efficiency.

Furthermore, the lower limit for SiO2 delivering good light transmissionrates is about the 200 nm level; light transmission drops dramaticallywith the shorter wavelength vacuum ultraviolet light that exhibitswavelengths lower than 200 nm. Furthermore, with very short wavelengthsof vacuum ultraviolet light in the 150 nm vicinity such as used withhigh energy fluorine lasers, not only does the foregoing lighttransmission rate decline, but the material cannot stand up to theapplication and the losing the clarity occurs.

Also, considering the fact that with synthetic silica glass, there is asignificant decrease in the transmission rate in the ultraviolet lightrange through window materials through which irradiated lamp light istransmitted, Japanese Patent Application Publication No Hei 8-315771(Patent Publication 5) discloses fluorine doping technology forsynthetic silica glass that aims at improving operational longevity.

However, using fluorine compounds to dope the silica glass base stockonly allows about a 50% transmission range in the 160-190 nm wavelengthranges, and it cannot stand up to use in the lower wavelengths of vacuumultraviolet light.

Accordingly, alkali halide materials, such as CaF2, LiF, MgF2, etc.,have generally been used for light transmitting window stock when vacuumultraviolet range ultraviolet light had to be transmitted.

A suitable example from the prior art is the aforementionedmicrowave-excited hydrogen ultraviolet lamp, which generated vacuumultraviolet light at a wavelength of 122 nm. The only known materialsthat could be used for light transmitting windows were CaF2, LiF, andMgF2, and since LiF and CaF2 exhibited dramatically lower lighttransmission from their color center, MgF2 was most commonly used.However, there has been no disclosure of any report that dealt withcountermeasures of losing the clarity for MgF2.

To wit, when the magnesium fluoride is used as the material for lighttransmitting windows, such windows exhibit a poorer longevity than otherwindow materials, and compared with lamps that use other windowmaterials, lamp longevity itself is only about half or less.

When using light with a higher photon energy than the absorptionwavelength for the material used in light transmitting window 8,especially light in the vacuum ultraviolet range, when the light fromthe discharge plasma is irradiated upon light transmitting window 8,said window 8 develops a defect, a so-called color center is producedthat lowers its light transmission rate. This phenomenon is also commonto CaF2, LiF, MgF2 and other alkali halide materials, and is caused bythe slight shift of fluorine atoms from their correct position withinthe lattice.

Further, the aforementioned conventional technology all addressedproblems associated with synthetic quartz, especially synthetic quartzoptical systems that used conventional wavelength ultraviolet light asthe light source. There have been no proposals for practical technologythat was effective in preventing the diminishment of the lighttransmission rate through MgF2, which is the material used in lighttransmitting windows for the 122 nm wavelength vacuum ultraviolet lightgenerated by microwave-excited hydrogen ultraviolet lamps.

Due to this situation, when the transmission rate declines, the only wayto deal with it is to replace the light transmitting window. In thisprior art, the degradation of light transmitting window 8 as describedabove was the determining factor in lamp longevity. In the prior art,once the lifespan of the lamp's light transmitting window 8 was up, itwould have to be replaced with a new light transmitting window torestore the light emission intensity of the lamp. The replacement oflight transmitting window 8 requires the breaking of the lamp's vacuumand several hours of labor, during which time, the lamp cannot be used.Further, during the replacement cycle, the output intensity from thelight source is constantly changing. Each time the transmitting windowis replaced, it requires calibration operations for the light intensity.Thus, it is difficult to use such lamps in applications that requirelong-term monitoring, such as employed in environmental measurements.

SUMMARY OF THE INVENTION

The present invention was developed after reflecting upon the problemsassociated with the prior art, and it relates to an apparatus and methodfor restoring the optical properties in a variety of apparatus thatemploy optical systems to deliver effects such as light transmission,diffraction, reflection, spectrum generation, and interference and thatuse high photon energy light such as conventional ultraviolet light orvacuum ultraviolet light. In particular, the objectives of the presentinvention are to prevent or inhibit the deterioration of optical systemsthat determine the longevity of the foregoing apparatus, and by sodoing, decrease the frequency of maintenance operations such as windowreplacement and to reduce the costs for such operations.

More specifically, according to the first preferred embodiment of thisinvention, the objective is to provide an apparatus and method for itsuse, which prevents or suppresses the degradation of said opticalsystems to thereby reduce the frequency of maintenance operations suchas replacement of the optical systems as well as reduce the costs forsuch operations, by preventing or suppressing the accumulation of carbonon the surface of optical systems such as the optical system providedoutside of light transmitting window 8 (for example, on outer surface 11of light transmitting window 8 shown in FIG. 14).

An additional objective of the present invention is to make it possibleto extend the longevity of optical equipment and improve the reliabilityof these apparatus by preventing or suppressing the accumulation ofcarbon on the irradiated surfaces and emission surfaces of opticalelements lying within the light path in a vacuum zone.

Further, according to the second preferred embodiment of this invention,the objective, after reflecting upon the above described issues in theprior art, is to provide an optical system and method for its use, invariety of apparatuses that use combination effects of lighttransmitting, refracting, reflecting, spectrum and interference, whichis provided in the light path of high photon energy light sources suchas plasma light and vacuum ultraviolet light. The optical system wouldsuppress the degradation of the optical equipment such as theaforementioned lenses, windows, etalons, prisms, reticles, reflectingmirrors and the like, all of which are provided inside of lighttransmitting window 8 (for example, on inner surface 10 of lighttransmitting window 8 shown in FIG. 14) to thereby maintain a stable andhigh intensity of light output over time, and to extend the longevity ofapparatus using various types of optical systems.

FIRST PREFERRED EMBODIMENT

To resolve the above described issues, the present inventor continuedresearch along the following lines.

Initially, in the first preferred embodiment, a detailed analysis of thedegradation that occurs outside of light transmitting window 8 facingvacuum zone 14 (such as an outer surface 11) was performed. Theapparatus employed in the experiments was that illustrated in FIG. 14,wherein light transmitting window 8 was attached via an O-ring to theplasma-exposed side of Flange 17. The analysis was performed on outersurface 11 of light transmitting window 8, to wit, the surface facingvacuum zone 14, which is the surface that emits the ultraviolet light.Since no deposits were found on inner surface 10 on the opposite side,no detailed analysis was performed on that surface.

Magnesium fluoride (MgF2) mono-crystal were used as the material forlight transmitting window 8, and the crystal axis (c-axis) was alignedto be perpendicular with the surface of the transmitting window. Crystalsize was 0.5 inch Φ×1 mm thick. The crystal was a UV grade product madeby Ohyo Koken Kogyo Co., Ltd. Several of such crystals from the same lotwere procured, and the crystals employed were matched for the quality ofthe crystal and the condition of their surfaces. The crystals wereanalyzed after their use in the lamp and all efforts were made toeliminate any error inducing factors due to any variation within thelot.

In the experiment, the first order of business was to use an opticalmicroscope to view outer surface 11 of light transmitting window 8 inthe central Φ8 mm region through which the ultraviolet light wastransmitted to observe any film-like material that might be adhering.Plastic forceps were then used to scrape at any adhering material, atwhich time it was discovered that a weakly adhering film of material wasadhering to outer surface 11.

Next, an elemental analysis was performed on the adhering material. EPMA(electron probe X-ray micro-analyzer) was used to perform an elementalanalysis of outer surface 11 of light transmitting window 8. (Analyticalconditions: acceleration voltage 15 kV, irradiation current 5E-8A,measurement methods: qualitative analysis, fine analysis, mappinganalysis.)

It was found as a result of the EPMA analysis that significant amountscarbon were detected on the central Φ8 mm region through which theultraviolet light had been transmitted. The donut shaped region outsideof the circular central Φ8 mm region through which the ultraviolet lightwas transmitted was in the shadow of Flange 17 and was a region throughwhich no ultraviolet light had passed, but contamination levels ofcarbon were detected in this donut shaped region. What is meant by a‘contamination level’ in this EPMA analysis would be the weak signal forcarbon detection that is generated even when analyzing a thoroughlycleaned surface. Thus, it is an unavoidable adherence of carbon thatgenerates such signals. Accordingly, the measurement limits for EPMAanalysis with regard to carbon is determined by the carbon contaminationlevel of the analytical apparatus. When the carbon signal level from thecentral Φ8 mm region was compared with the contamination signal level,the former was found to be significantly higher, which confirmed thefact that a film-like accumulation of carbon had occurred on outersurface 11 of the light transmitting window.

As has been previously stated, the mechanism for the carbon buildup,with reference to the apparatus shown in FIG. 14, involves organic gasesbeing present in vacuum zone 14, and when these organic gases areabsorbed on outer surface 11 of light transmitting window 8, and thenwhen vacuum ultraviolet light is transmitted through light transmittingwindow 8, the organic gases undergo a dehydrogenation reaction toconvert them to carbon, which accumulates upon outer surface 11.

As the use of light transmitting window 8 in the above-describedenvironment continues, so does the accumulation of carbon, which, withtime, reduces the rate of light transmission. Accordingly, sincecompared with its initial state, the transmission rate for lighttransmitting window 8 becomes reduced, some mechanism to remove thisaccumulated carbon from outer surface 11 is clearly required. Since itwas found that the film-like accumulation of carbon upon outer surface11 was the primary cause for the degradation of the light transmittingwindow, the present inventor continued research on counter measures,which lead to the below described completion of the present invention.

The present inventor, experimentally confirmed the below describedapproach to the problem. The raw material for the carbon is the organicgases, but it is virtually impossible to completely eliminate them.Further, if they are not irradiated by vacuum ultraviolet light, then nodehydrogenation reaction occurs, but then the apparatus would not beable to carry out its functions as a light emitting apparatus. Thelocation of the carbon deposits exactly matches the position throughwhich the vacuum ultraviolet light is irradiated. The vacuum ultravioletlight directly excites the organic gases to force the dehydrogenationreaction, but such high photon energy does not just excite organicgases, but many types of molecules can be so excited and put into anactive state.

With reference to FIG. 1 as an example, the light output apparatus inthis example is a 122 nm wavelength vacuum ultraviolet light outputusing hydrogen light emission, and the photon energy for this vacuumultraviolet light is 10.2 eV. This level of photon energy will exciteoxygen gas, H2O gas (steam) and can generate radicals having strongoxidizing power. The reason for maintaining vacuum zone 14 is becausethe oxygen, carbon dioxide, water vapor and other components of theatmosphere would absorb the vacuum ultraviolet light and weaken itsintensity. Accordingly, the absorption medium, i.e. the atmosphericcomponents, are eliminated by means of a vacuum pump, etc. to createvacuum zone 14.

However, even though these were atmospheric components, since theycontained O2, water vapor and the like, it was found that by loweringtheir concentration appropriately (reducing their pressure), it waspossible to generate radicals having oxidizing power withoutdramatically attenuating the vacuum ultraviolet light. When the lightoutput apparatus was operated under conditions whereconcentration-adjusted atmospheric components coexisted and when thelater stage vacuum zone 14 was implemented, it was possible to removethe carbon adherents on outer surface 11 of light transmitting window 8.Further, it was possible to remove the carbon adhering to the surface ofall the optical elements located in vacuum zone 14. The reason why itwas possible to remove the carbon in this way is that the carbonadherence to outer surface 11 of light transmitting window 8 was takingplace at the same time as the carbon decomposition and removal by theradicals, and the rate at which the radicals were decomposing andremoving the carbon exceeded its formation rate.

Since the decomposition reaction of the carbon using radicals convertedthe carbon into volatile molecules such as carbon dioxide and watervapor, these could be removed rapidly from the system using the vacuumpump. The radicals that are created in this case are elemental oxygenand ozone generated by the excitation of oxygen molecules, and OHradicals produced through the excitation of water vapor, etc.

Furthermore, when the light output apparatus is operated in the presenceof these concentration-adjusted atmospheric components and when therewere preexisting carbon deposits on outer surface 11 of lighttransmitting window 8, there was a gradual decomposition and removal ofthat carbon so that finally, it was possible to completely remove all ofthe carbon and restore light transmission window 8 to its original lighttransmission rate. Then, the light output apparatus was operated withoutoperating the later stage vacuum zone function using optical elementsalready having carbon accumulation, and it was possible to remove thecarbon from the surface of these optical elements that had been locatedin vacuum zone 14.

Accordingly, using the findings from this invention, it is possible toprevent the degradation of the transmission window of light outputapparatus in order that the light intensity produced by said lightoutput apparatus not be diminished, to thereby make it possible to notonly eliminate the maintenance costs associated with replacement of thelight transmitting window and maintenance down time for the equipment,but it is further possible by means of operating the light outputapparatus to remove carbon deposits already formed on light transmittingwindow 8 or optical elements and to restore them to their original stateto thereby recover full performance in the vacuum zone and to sustainvacuum zone 14 with reduced maintenance costs and maintenance frequency.

Next, the method used to adjust the concentration of the atmosphericcomponents in the vacuum zone 14 environment will be explained. The gassupply can employ pure oxygen gas supplied from an oxygen cylinder.Alternatively, it can be drawn from the air, or, an air line alreadyinstalled in the factory may be used. A gas cylinder filled with driedair may be used as well. It is further possible to use mixtures ofoxygen and an inactive gas such as argon, helium or the like which aredispensable from gas cylinders. The pressure of the gas supply, as willbe explained later in the configuration examples, may be controlled bythe aperture of the valve and the ability of the gas cylinder to purgevacuum zone 14 and control the partial pressure of the gases.

It is further possible to add water vapor to the gases described above,or to use only water vapor. Water vapor may be added by means ofpreparing a container with water sealed within and have one area filledto saturation with water vapor, and then by mixing this water vapor withany of the above described gases. When used by itself, it need only tobe introduced into vacuum zone 14. The temperature of the water may beroom temperature, or it may be chilled or heated. The pressure of watervapor at the saturation level varies according to the temperature of thewater, so it is possible, as will be described later in theconfiguration examples, to set the valve aperture and pump purging powerto control the partial pressure of the water vapor in vacuum zone 14.

The partial pressures of the aforementioned gases in vacuum zone 14 aredetermined by the below described conditions. The upper limit for thegas partial pressure should be determined based upon the objective forabsorption function for the vacuum ultraviolet light by the gases and bypartial pressure that would not impede operations. In specific terms,for example, if it is oxygen gas, the upper limit should essentially beon the order of 10 mtorr (under 20 mtorr). Should that partial pressurelevel be exceeded, the absorption of the vacuum ultraviolet light by theoxygen would reach the point where it could not be ignored—it wouldbegin to impede the functional objectives for vacuum zone 14. However,should the length of the light path through vacuum zone 14 besufficiently short, it would be possible to ignore the effects of theoxygen concentration at higher upper limits. The accurate setting of theupper limit value can be determined by filling the vacuum zone 14 with aspecified partial pressure of gas and checking whether the functionalobjectives have been impeded. Specifically, the amount of the vacuumultraviolet light can be measured as a means of investigating the levelof its attenuation.

The lower limit for the gas partial pressure should be set to be abovethe processing capability for the load at hand. Here, what is meant by‘load’ is the rate at which carbon accumulates on outer surface 11 oflight transmitting window 8 as determined by the type and concentrationof the organic gases present, and the wavelength and intensity of thevacuum ultraviolet light 9 that is transmitted through lighttransmitting window 8. What is meant by ‘processing capability’ is therate at which the carbon can be decomposed and removed by the radicalsgenerated through the excitation of the gas by the vacuum ultravioletlight.

Experiments were performed by the present inventor that envisagedvarious cases for vacuum zone 14 in which there were some unknowns, butfor example, in the case of vacuum zone 14 produced by a conventionalturbo-molecular pump and using a dry pump exhaust system, the lowerlimit for oxygen gas, for example, would be about 0.01 to 0.1 mtorr. Ifthis level of oxygen gas exists, then it will exhibit adequateprocessing capability for the load. Water vapor provides a relativelyhigher level of processing capability than oxygen, and its virtual lowerlimit would be on the order of 0.005 to 0.01 mtorr.

To accurately set this lower limit, oxygen can be used to fill thevacuum zone 14 that is in actual use to a certain partial pressure andthen the light output apparatus can be operated followed by analysis forany adherents on outer surface 11 of light transmitting window 8.Appropriate methods of analysis include observation under an opticalmicroscope or carbon analysis using EPMA. So long as the analysis doesnot reveal the presence of significant carbon deposits, the partialpressure that was used at that time can be confirmed to be on thatallows the adequate decomposition and removal processing of the carbon.

Now, the below described technological means is proposed for thisinvention on the basis of the foregoing findings.

The invention relates to an apparatus which can attain the effects ofthe present invention. The present invention provides an opticalproperties restoration apparatus for improving the reliability andlongevity of optical properties of an optical system by preventing,suppressing, or improving degradation of optical properties of anoptical system lying in output light or along a light path of saidoutput light, wherein said optical system is provided within a nearvacuum zone where organic components may be decomposed, said degradationis caused by carbon deposited or accumulated upon light irradiationsurfaces, light reflection surfaces, light emission surfaces(collectively called ‘lighting surfaces’) of said optical system, andsaid surfaces faces said vacuum zone, said optical propertiesrestoration apparatus comprising: a means to create a near vacuum zonewith a presence of active energy to excite an oxidation reaction ofcarbon, said near vacuum zone facing said lighting surfaces of saidoptical system; a means to generate negative ions or radicals in saidnear vacuum zone; a means to facilitate an oxidation reaction betweensaid negative ions or radicals and said carbon in said near vacuum zone;and wherein said optical properties restoration apparatus removes orreduces the accumulated carbon which deposits on said lighting surfaceby the oxidation reaction.

More specifically, it includes: a means to create a near vacuum zone toexcite an oxidation reaction of carbon, said near vacuum zone facingsaid lighting surfaces of said optical system; a means to generate aflow of an oxygen atom-containing gas such as water gas or oxide gas insaid near vacuum zone; a means to supply active energy in said nearvacuum zone to cause a carbon oxidation reaction between said oxygenatom-containing gas and the carbon; and wherein said optical propertiesrestoration apparatus removes or reduces the accumulated carbon whichdeposits on said lighting surface by the oxidation reaction.

“Near vacuum zone” mentioned above can be defined by a vacuum space inwhich high active energy excitation excites a carbon oxidation reactionto decompose the carbon by eliminating hydrogen from the organiccompound of hydrocarbon and etc. The pressure of the near vacuum spacefluctuates by the strength of active energy and the oxidizing power ofthe oxygen atom-containing gas mentioned above, however it must be underseveral tens mtorr.

The invention further relates to an optical properties restorationmethod for improving the reliability and longevity of optical propertiesof an optical system by preventing, suppressing, or improvingdegradation of optical properties of an optical system lying in outputlight or along a light path of said output light, wherein said opticalsystem is provided within a near vacuum zone where organic componentsmay be decomposed, said degradation is caused by carbon deposited oraccumulated upon light irradiation surfaces, light reflection surfaces,light emission surfaces (collectively called ‘lighting surfaces’) ofsaid optical system, and said surfaces faces said vacuum zone, saidoptical properties restoration method comprising steps of: creating anear vacuum zone with a presence of active energy to excite an oxidationreaction of carbon, said near vacuum zone facing said lighting surfacesof said optical system; generating negative ions or radicals in saidnear vacuum zone; and removing or reducing the accumulated carbon whichdeposits on said lighting surface, by reacting the deposited carbon withthe negative ions or radicals.

More specifically, the method is characterized by steps of: creating anear vacuum zone to excite an oxidation reaction of carbon where highactive energy excitation exists, wherein said near vacuum zone facessaid lighting surfaces of said optical system; and supplying activeenergy while supplying a flow of gases containing oxygen atoms (e.g.water vapor, oxygen, hydrogen peroxide, ozone or mixtures of them withinactive gases (including air)), such as water gas or oxidizing gas intosaid near vacuum zone, thereby removing or reducing the accumulatedcarbon which deposits on said lighting surface by exciting the oxidationreaction of the accumulated carbon with said supplied active energy.

Further, the foregoing optical system includes not just an opticalelements comprised of light transmitting or reflecting members that arelocated on the boundaries of the near vacuum zone, but also the opticalcomponents comprised of diffraction, refraction, spectrum generation,transmitting and diffraction adjustment optical elements lying along thelight path inside the vacuum zone, and the optical articles to besurface treated by the irradiated light, and further said optical systemincludes the position adjustment and retention mechanisms, containers,and seals of said optical elements or said optical articles as well.

Also, the invention may be effectively applied even in cases where thebeam that forms said light path is normal ultraviolet light having awavelength of 380 nm or under, or, vacuum ultraviolet light having awavelength of 200 nm or under, and said optical system outputting saidultraviolet light or lying along a light path of said output light is anoptical materials comprising one or combinations of fluoride compoundssuch as magnesium fluoride, calcium fluoride, barium fluoride, aluminumfluoride, Cryolite, Thiolite or other fluoride compounds, metalfluorides such as lanthanum fluoride, cadmium fluoride, neodymiumfluoride, yttrium fluoride, or, high purity oxides such as syntheticquartz glass or sapphire.

Additionally, a lower limit value for a partial pressure of said gasescontaining oxygen atoms that is supplied to said near vacuum zone ispreferably set to a level over a speed of the carbon buildup, in caseswhere the carbon from the decomposition of organic components in saidnear vacuum zone has already grown on the surfaces of said opticalsystems and opposing surfaces. Further, an upper limit value for apartial pressure of said gases containing oxygen atoms that is suppliedto said near vacuum zone is preferably set to below the level where theabsorption of vacuum ultraviolet light by said oxygen atom-containinggas cannot be ignored from the perspective of its performing itsfunction inside said near vacuum zone, in cases where said opticalproperties of said optical system is restored when said vacuumultraviolet light is irradiated to said optical system or irradiatedfrom said optical system.

The upper limit value for the partial pressure of the oxygenatom-containing gas is preferably set to a level by actually fillingsaid near vacuum zone with a certain partial pressure of the oxygenatom-containing gas, and then measuring the quantity of vacuumultraviolet light on said light path to check its attenuation level.

Furthermore, when the aforementioned oxygen atom-containing gas isoxygen gas, the range between the lower limit-upper limit for the gaspartial pressure is 0.02 mtorr-20 mtorr (preferably 0.02 mtorr-10mtorr), and when the gas is water vapor, 0.01 mtorr-10 mtorr (preferably0.01 mtorr-1 mtorr).

Also, this invention is effective when the beam formed on the foregoinglight path has a high photon energy and is a beam of a specificwavelength in the vacuum ultraviolet light wavelength range.

To wit, so long as the foregoing active energy is vacuum ultravioletlight with a high photon energy, the negative ions or radicals may begenerated from the oxygen atom-containing gas without using a separatesource of energy (e.g. heat energy, plasma energy, electrical energy,etc.) Although active seeds such as OH— and O— that were cited in PatentPublication 3 are employed, the conditions for their pressure withrespect to the object of the cleaning differs from those specified inthat referenced document.

SECOND PREFERRED EMBODIMENT

Further, in the second preferred embodiment, a detailed analysis of thedegradation that occurs inside of light transmitting window 8 (such asan inner surface 10) was performed.

To resolve the foregoing problems, the present inventors performeddetailed analyses of the degradation of light transmitting windows 8made from fluoride materials. The device we used is shown in FIG. 14,wherein light transmitting window 8 is attached to the plasma-exposedside of flange 17 via an O-ring. MgF2 (magnesium fluoride) mono-crystalwas used to fabricate light transmitting windows 8.

As a result, it was found that after irradiation by vacuum ultraviolet,the degradation in the transmission rate of light transmitting window 8from the irradiation of vacuum ultraviolet was caused by the formationof an oxide on the surface of the MgF2 crystals (several tens of nmthick). It was also confirmed that there was a reduced presence offluorine in this region that was several tens of nm thick on the surfaceof the crystal.

Further, by measurements of the spectral transmission rate toinvestigate any corresponding relationship that might exist between thegeneration of a color center on light transmitting window 8 and thedegradation of the light transmission rate, it was found that the maincause of the degradation of light transmitting window 8 was notabsorption by the color center, but by the defects in the fluorinegenerated in the surface of the crystal and the presence of oxygen.

At that point, the below described technological means was proposed forthis invention that focused on the foregoing findings.

The first proposal for this invention related to optical systemscomprised of fluoride materials characterized in that a protective filmhaving a film thickness of 2-20 nm is formed at least on the lightirradiation side (inner side) of said optical system to prevent thestripping of the fluorine atoms from the surface of said optical system.

The differences between the present invention and Patent Publication 4will now be explained. Patent Publication 4 related to mercury dischargelamps having mercury sealed within their arc tube. The technologyprevented the affixation of the mercury to the inner wall of the arctube by using a 0.1 μm to 0.15 μm protective film of alumina, etc.

On the other hand, the present invention relates to the vacuumultraviolet light range, wherein a very thin film of 2 nm-20 nm isapplied to surfaces irradiated by vacuum ultraviolet light to preventthe stripping off of fluorine, this in exchange for the initialdegradation in optical properties caused by the coating.

The reason for limiting the film thickness to 20 nm or under is that ifit were any thicker, it would absorb the vacuum ultraviolet light to thepoint where it would not be able to maintain its function as an opticalelement.

The lower limit of 2 nm or over is that required to assure a uniformcovering of the protective film upon the crystal surface. Since themolecular diameter of SiO2 or Al2O3, MgO, TiO2, or ZrO2 is approximately1 nm, if the coating is not at least 2 molecules thick, it is notpossible to deliver a uniform protective film over the surface of thecrystal to achieve the function of the present invention.

When the film thickness is adequate, although the aim is to protect thesurface of the optical system, since metal oxides such as SiO2 or Al2O3,MgO, TiO2, ZrO2 are not materials that inherently allow vacuumultraviolet light to pass through, the existence of such protectivefilms allows the absorption of the vacuum ultraviolet light within thefilm, and as shown in FIG. 13, diminishes the amount of ultravioletlight passing through to the base material. At the 20 nm thicknesslevel, the transmission rate is only 10% of what it would be with nofilm. Not only would initial transmission rates of under 10% causemassive degradation of the optical properties of the base material, butat that level of degradation it could not function as an optical systemand there would be the concern that the absorption of the ultravioletlight would degrade the protective film itself and that heat would causeit to peel off from the surface of the optical system or cause otherdamage. Accordingly, a thickness of 12 nm or less, preferably 10 nm orless, will maintain 30 to 40% of the optical properties of the basestock, and even in worst case scenarios, would maintain the opticalproperties at 10% or higher. Accordingly, due to absorption ofultraviolet light, film thicknesses of over 20 nm could not be expectedto deliver the functions desired of the optical system.

Also, the oxidation of Mg occurs along with the stripping of thefluorine atoms, so a protective film of SiO2 or metal oxides having afilm thickness of 2-20 nm, preferably 2-12 nm, even more preferably 2-10nm, is preferably formed at least on the light irradiation side (innerside) of said optical system to prevent the stripping of the fluorineatoms from the surface of said optical system.

With the present proposal, it is possible to suppress both the strippingof fluorine atoms and oxidation of the surface of the foregoing opticalsystems, to thereby inhibit the degradation of the light transmissionrate of the optical systems.

There are growth methods in the gaseous phase such as vapor deposition,ion plating, CVD and the like which can be used to form the thin-filmprotective film, but the especially preferred methods of film formationare the ion beam sputtering method and plasma CVD, because such methodscan create a very uniform film thickness that follows along thedepressions and protrusions created by the polishing process for theoptical systems.

The second proposal of the present invention relates to an opticalsystem comprising fluoride compound having surfaces facing and exposedto plasma installed in an optical equipment which has an inner zonewhere the plasma exists, wherein a 2 nm-20 nm protective film of ahighly plasma-resistant material is formed on the surface of saidfluoride compound that is exposed to the plasma.

This proposal enables the suppression of the degradation of the lighttransmission rate of the optical system by means of suppressing thestripping of fluorine atoms or the oxidation of the surface of theforegoing optical system otherwise caused by o the plasma environment.

In this case, the foregoing protective film may be any of the metaloxides such as SiO₂ or Al₂O₃, MgO, TiO₂, ZrO₂, which were cited above asprotective films. By forming the foregoing protective film on theoptical system, wherein the optical system comprises of mono-crystalfluoride material having the crystal axis (the c axis) along thedirection of the light irradiation, and the perpendicular surface ofsaid protective film is coated by SiO₂ or metal oxides, it is possibleto prevent the deterioration over time of the base stock from the vacuumultraviolet light irradiation, with the initial degradation of theforegoing fluoride optical system from the coating being the trade-off.

Also, since the SiO₂ or other of the above cited metal oxides have ahigher resistance to plasma than the fluoride compounds, it is possibleto inhibit the stripping of fluorine or oxidation of metal atoms, andsince they are oxides themselves, as a result, when used as a protectivefilm for optical systems made of fluoride materials, after their initialdegradation, it is possible to prevent the further degradation over timeof the mother stock otherwise caused by its irradiation by vacuumultraviolet light.

The third proposal of the present invention relates to a method to usethe devices, that use the optics of the foregoing optical systems andare characterized in that applying in advance a protective film of 2nm-20 nm of a metal oxide selected from SiO₂ or Al₂O₃, MgO, TiO₂, ZrO₂to an optical system, wherein said film suppresses the stripping off ofa structural element from the surface of the base stock or the oxidationof the surface of the base stock, by the irradiation of vacuumultraviolet light over time or the plasma exposure to the base stock,and incorporating said optical system into a desired device that hasvacuum ultraviolet light sources or plasma light sources which hashigher photon energy than an absorption wavelength of a base stock ofsaid optical system.

According to this invention, after the initial degradation in propertiesdue to the foregoing metal oxide protective film, it is possible bymeans of this film to suppress the deterioration over time of the basestock of optical systems due to the stripping of elements from the basestock or oxidation of the surface of base stock that are caused byirradiation of vacuum ultraviolet light or exposure to plasma, whichmeans that the optical output from the base stock will not be furtherdiminished after operations are first initiated, which, for examplemakes it possible to extend the longevity of light transmitting windowsof reflecting mirrors of the foregoing light output devices. This alsoextends the interval between replacement maintenance for the lighttransmitting windows or reflecting mirrors to thereby improve theoperation rate for the light output devices and decrease their operatingcosts.

In this case, it is necessary only to use a light source that willprovide adequate light output to compensate for the initial degradationof the optical system due to the foregoing protective film as a means tomake it possible to increase the longevity of the device, to thereby notcause the reduced light transmission of the overall system (transmissionrate, reflection rate).

To wit, when the optical system is coated with the foregoing protectivefilm in advance to suppress its deterioration over time due toirradiation by vacuum ultraviolet light or exposure to plasma, whichwould otherwise cause elements from its base stock to be stripped awayor oxidized over time, it is only necessary to supplement the lightoutput for the device to compensate for the initial deterioration thatis caused by the foregoing protective film. For example, in a lightoutput device which is used as a light source for measurements, by usingthe above described optical systems coated on at least one side, such ascoated light transmitting windows or reflecting mirrors, it is possibleto obtain stable light output over the longer term, and to employ thelight output device for measurement applications and maintain stablelight transmission rates that do not deteriorate so as to stabilize thecontrol operations and measurement sensitivity of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the structure of a microwave excitedhydrogen ultraviolet lamp used to explain the first example of the firstpreferred embodiment according to the present invention.

FIG. 2 is a diagram showing the structure of a microwave excitedhydrogen ultraviolet lamp used to explain the second example of thefirst preferred embodiment according to the present invention.

FIG. 3 is a diagram showing the structure of a microwave excitedhydrogen ultraviolet lamp used to explain the third example of the firstpreferred embodiment according to the present invention.

FIG. 4 is a diagram showing the structure of a microwave excitedhydrogen ultraviolet lamp used to explain the forth example of the firstpreferred embodiment according to the present invention.

FIG. 5 is a diagram showing the structure of a microwave excitedhydrogen ultraviolet lamp used to explain the fifth example of the firstpreferred embodiment according to the present invention.

FIG. 6 is a diagram showing the structure of a microwave excitedhydrogen ultraviolet lamp used to explain the sixth example of the firstpreferred embodiment according to the present invention.

FIG. 7 is EPMA analytical results for carbon adhesion to the lighttransmitting window according to the first preferred embodiment of thepresent invention.

FIG. 8 is a diagram showing the structure of a microwave excitedhydrogen ultraviolet lamp used to explain the second preferredembodiment according to the present invention.

FIG. 9 is a graph showing the XPS depth distribution measurements for alight transmitting window before its being used that does not have aprotective film coating.

FIG. 10 is a graph showing the XPS depth distribution measurements for alight transmitting window after being used that does not have protectivefilm coating.

FIG. 11 is a graph showing the XPS depth distribution measurements for alight transmitting window before its being used that has a protectivefilm coating.

FIG. 12 is a graph showing the XPS depth distribution measurements for alight transmitting window after being used that has a protective filmcoating.

FIG. 13 is a graph showing the relationship between the lighttransmission rate of an optical system in its initial state with thatshowing various thicknesses of a protective film.

FIG. 14 is a diagram showing a conventional microwave-excited hydrogenultraviolet lamp according to the prior art.

DETAILED DESCRIPTION OF THE INVENTION First Preferred Embodiment

The first Preferred embodiment implementations for the present inventionin suppressing or removing carbon adherents from outer surface 11 oflight transmitting window 8 shall be described in the Examples belowwith reference to the figures. In addition, preferred embodiments forsuppressing or removing carbon adherences from optical systems locatedin vacuum zone 14 will be explained as well with reference to thefigures.

The present invention is not confined to these embodiment examples, andit may also be effectively applied to lamps or laser apparatus thatproduce light by electrical discharge or by heating.

EXAMPLE 1

FIG. 1 is a diagram used to explain the structure of themicrowave-excited hydrogen ultraviolet lamp used in the first example ofthe first preferred embodiment according to the present invention.

The retaining member (flange) 17 where light transmitting window 8 isattached is disc shaped and its center is aligned with the bore ofdischarge tube 1, and it contains an opening which is of a largerdiameter than the inside diameter of the discharge tube. Window flange17 includes an O-ring groove to create a seal over the opening for lighttransmitting window 8, and there is also a hollow lid-shaped jig 20,bolt holes to affix it, and an O-ring groove which connects to dischargetube 1 to maintain a vacuum with window flange 17.

The internal structure of jig 20 employs two-stage concentric circlesand bounds the space for housing light transmitting window 8 and thespace encompassed by discharge tube 1. On the end that encompassesdischarge tube 1 the face has been cut to an angle where O-ring 13 isheld in place by pressure. Threads not shown in the figure are furthercut into the outside circumferential surface of this end, and the vacuumboundary for discharge tube 1 is formed by tightening cap 21 over thecylindrical opening with a seal by O-ring 13. Window attachment Flange17, jig 20 and cap 21 are all made from metal; in general, lowcontamination stainless steel or aluminum would be used, but thematerial is not limited to these metals.

The operation of the microwave-excited hydrogen ultraviolet lamp of theabove described structure will now be explained. First, from thedischarge gas supply opening 2 in discharge tube 1, hydrogen dischargegas diluted with helium at 1/100 is fed in at 20 sccm. The discharge gasis expelled through exhaust opening 3 by a vacuum pump (not shown). Byadjusting the aperture of a valve (not show) installed between dischargegas exhaust opening 3 and the vacuum pump, it is possible to adjust theexhaust conductance to maintain the inside of discharge tube 1 at about5 torr (665 Pa). The reason for creating the flow of discharge gas inthe direction from the light transmitting window side toward dischargetube 1 is to do everything possible to reduce sources of contaminationon said window 8 due to materials being generated inside discharge 1 bythe discharge plasma.

Next, 2.45 GHz, 50 W microwaves are supplied from microwave supplyconnector to microwave oscillator 4. The microwaves may be suppliedeither continuously or intermittently. A regulator (not shown) installedin the electrical power line connected to the microwave power source andmicrowave oscillator may be used to adjust the microwave power outputbetween the power source and load (discharge plasma) in generatingdischarge plasma 7 in discharge tube 1. The hydrogen atoms excited bydischarge plasma 7 irradiate light in the 103 nm and 122 nm vacuumultraviolet light wavelengths. Since MgF₂ was used as the material forlight transmitting window 8, as will be detailed below, the 103 nm lightrays are absorbed by the MgF₂ and only the 122 nm wavelength vacuumultraviolet light is passed into vacuum zone 14 as output lamp light(vacuum ultraviolet light) 9.

In this case the opening in the mounting Flange 17 for lighttransmitting window 8 is Φ8 mm, so the output into vacuum zone 14 is Φ8mm flux of light.

MgF₂ (magnesium fluoride) mono-crystal was used for light transmittingwindow 8, with the crystalline axis (c axis) aligned to be perpendicularto the surface of the light transmitting window. The crystal size was0.5 inch Φ(12.7 mm Φ)×1 mm thick. The crystal used was UV grade fromOhyo Koken Kogyo Co., Ltd. A plurality of crystals from the same lotwere obtained, and they were sorted to match their crystal quality andsurface condition to eliminate any variation within the lot to thedegree possible in order to be able to verify just the effects of theprotective film.

Also, photodiode 12 was positioned to receive lamp output light 9 as ameans of monitoring the amount of light output from said lamp.

Oxygen gas was supplied to vacuum zone 14 using the below describedmethod while regulating the gas to the prescribed partial pressure.

Oxygen gas cylinder 23 (made by Nippon Sanso Corporation) was filledwith pure oxygen (purity 4N) and connected to regulator 22. Afteradjusting the gas pressure to 0.1 kg/cm², and adjusting the aperture ofvariable leak valve 19 connected via pipe 16 c, the gas passed throughpipe 16 b on the atmospheric side, and then passed via seal mechanism(not show) and was fed into vacuum zone 14 from pipe 16 a inside ofvacuum zone 14. The amount supplied was approximately 1 sccm. Vacuumzone 14 was evacuated by means of a turbo molecular pump (evacuationrate of 50 L/min, model TP-50 made by Mitsubishi Heavy Industries,Ltd.), and was connected downstream to a dry pump (not shown). In thiscase, the oxygen gas partial pressure inside the vacuum zone wasbalanced at 1 mtorr (1 millitorr). Thus, the conditions were such thatthe partial pressure of the oxygen gas inside vacuum zone 4 was on theorder of at least 1 mtorr (and under 10 mtorr).

Experiments were also performed with the valve aperture adjusted toprovide 5 mtorr, 2 mtorr and 0.1 mtorr, but as will be explained later,similar effectiveness was obtained for carbon removal.

The variable leak valve mentioned in the explanation is not an item withspecial specifications; it is merely a mechanism to make fine apertureadjustments, and any such mechanism of whatever name may be employed.

Next, photodiode 12 was employed to measure the changes over time of theamount of light output from the microwave-excited hydrogen ultravioletlamp of the aforementioned structure.

First, discharge plasma 7 was used to excite the hydrogen atoms togenerate the vacuum ultraviolet light for 90 hours (about 4 days). Next,as a control, the test was repeated but without the oxygen gas supply,to wit, the foregoing turbo molecular pump was operated to maintain asimilar environment (0.001 mtorr), and then the results were compared.

The results indicated that when the oxygen gas was fed during lampoperations, there was no observable degradation of the transmissionthrough light transmitting window 8 rate due to carbon accumulation. Onthe other hand, in the control, if the original light transmission rateis taken to be 100%, the transmission rate over the course of the testfell to 35% due to the accumulation of carbon on light transmittingwindow 8.

FIG. 1 shows the carbon 15 that was observed in the control experimentto accumulate and adhere in a film like manner. When the lamp wasoperated with a flow of oxygen gas, the carbon 15 shown in FIG. 1 didnot adhere to light transmitting window 8.

When light transmitting windows 8 were observed with an opticalmicroscope after having been used, no adherents were noted on the oneused with the oxygen gas feed, but in the control sample, materialadhered in a film-like manner over the central Φ8 mm range through whichthe vacuum ultraviolet light was transmitted. It was possible to peeloff the adhering material by scraping plastic forceps across outersurface 11, and the material was found to be a film like material withweak binding force adhering to outer surface 11.

Next, an elemental analysis was performed on the adhering material.Elemental analysis was performed on outer surface 11 of lighttransmitting window 8 for the control sample using EPMA (electron probeX-ray micro analyzer (the JXA-8200 made by Nippon Denshi usinganalytical conditions of acceleration voltage 15 kV, irradiation current5E-8A, measurement methods: qualitative analysis, line analysis, andmapping analysis. Results indicated that there was significant carbondetected in the central Φ8 mm area of outer surface 11 of lighttransmitting window where the ultraviolet was transmitted. The ringshaped area outside the central Φ8 mm region was in the shadow of Flange17, and accordingly, was a region through which no ultraviolet light wastransmitted, and although EPMA analysis revealed contamination levelcarbon in this area, there was no significant carbon adherence. What ismeant here by ‘contamination level’ in the EPMA analysis is just a weaksignal level for carbon such as obtained when analyzing a thoroughlycleaned surface. The act of irradiating a clean surface with an electronbeam unavoidably causes carbon to adhere, and this signal level is basedupon that adhering carbon. Accordingly, the contamination level of theanalytical apparatus itself determines the lower measurement limit forEPMA analysis. The signal level from the central Φ8 mm range throughwhich the ultraviolet light was transmitted, when compared to the signallevel for contamination, was significantly higher, and that findingconfirmed that carbon had accumulated on outer surface 11 of the lighttransmitting window in a film-like manner.

FIG. 7 shows the results of line analysis of the control experimentusing EPMA. The units on the horizontal axis in FIG. 7 are millimeters,which express the analytical position upon the diameter of the MgF₂crystal; the line analysis on said crystal was performed fromedge-to-edge.

The vertical axis expresses the carbon signal strength detected at thespectrum generation crystal LDE2. The main analytical conditions arelisted outside of the graph of FIG. 7.

From FIG. 7, it is apparent that there was a significantly high signalstrength from the carbon in the Φ8 mm region through which theultraviolet light was transmitted, which clearly indicated the film likeadhesion in the central Φ8 mm region.

On the other hand, no significant carbon signals beyond thecontamination level were detected from the surface of the lighttransmitting window after the lamp was operated under an oxygen flow.

As described above, by operating the lamp with an oxygen gas feed, itwas possible to prevent or suppress carbon buildup on light transmittingwindow 8.

Implementing this countermeasure makes it possible to suppress thedecline of the transmission rate through the light transmitting windowto thereby reduce the cost of maintenance operations to replace thewindow as well as reduce the operational down time for the lamp.

This embodiment took up the light transmitting window as an example, butthe present embodiment may be similarly applied to apparatus employinglight reflecting mirrors (windows). Examples of such light reflectingmirrors are those reflecting mirrors used with laser oscillators andlamp focusing mirrors. The embodiments described below also similarlyapply to the case of light reflecting mirrors.

EXAMPLE 2

FIG. 2 is a diagram showing a microwave excited hydrogen ultravioletlamp that will be used to describe the second example of the firstpreferred embodiment according to the present invention. Furtherelaboration of structural and operational elements that are similar tothose of Example 1 will be omitted. The specifications of lighttransmitting window 8 were the same as those explained for Example 1.Further, a photodiode 12 was positioned to receive the light output oflamp emitted light 9 as a means of monitoring the amount of light outputfrom said lamp.

Water vapor was supplied to vacuum zone 14 using the following method,and it was adjusted to a specific gas partial pressure. Glass tube 24(tube diameter Φ6 mm), which was filled with 1 mL of water 25 (purewater that was distilled, ion-exchanged processed and filtered) wasconnected with tube 16 d via flange 17. The structure of flange 17incorporated an O-ring to seal the glass tube off from the atmosphere,and all of the atmospheric components were exhausted from the tube inadvance. Water 25 was maintained at room temperature (25° C.) and theinside vapor pressure was 24 torr (computed value). This vapor pressurewas supplied at its primary vapor pressure via tube 16 d, and afteradjusting the aperture of variable leak valve 19, it passed through tube16 b on the atmosphere side, via a seal mechanism (not shown), and intovacuum zone 14 via tube 16 a. The amount of the supply was approximately0.1 sccm. Vacuum zone 14 was evacuated by means of a turbo molecularpump (evacuation rate 50 L/min, model TP-50 made by Mitsubishi HeavyIndustries, Ltd., not shown in figure), and dry pump (not shown)downstream. In this case, the water vapor partial pressure was balancedinside the vacuum zone at 0.1 mtorr (0.1 millitorr). Accordingly,conditions were such in vacuum zone 14 that the water vapor partialpressure was at least on the order of 0.1 mtorr, (but less than 1mtorr).

Experiments were also performed using water vapor partial pressures of 1mtorr and 0.01 mtorr, which where achieved by adjusting the valveaperture, but as will be described later, similar effects in carbonremoval were obtained.

Next, the changes over time of the amount of light output from theabove-described microwave excited hydrogen ultraviolet lamp weremeasured using photodiode 12.

First, hydrogen atoms were excited by discharge plasma 7 to generatevacuum ultraviolet light for 90 hours (about 4 days). Next, the lamp wasoperated without supplying the water vapor, to wit, the experiment wasimplemented using the foregoing turbo molecular pump to maintain anpressure environment of 0.001 mtorr, and then the results of the twotests were compared.

Results indicated that when the lamp was operated with a supply of watervapor, no degradation in transmission through light transmitting window8 due to carbon buildup could be observed. On the other hand, in thecontrol experiment, if the initial level of light transmission was ratedat 100%, the adhesion of carbon caused the transmission rate to drop to35% over the period measured.

The carbon 15 shown in FIG. 2 shows the film-like adhesion of carbonthat was observed in the control, but when water vapor was suppliedduring lamp operation, there was none of the adhesion of carbon 15 onlight transmitting window 8 as shown in FIG. 2.

When outer surface 11 of light transmitting window 8 was observed usingan optical microscope after it had been used, no adherents were observedfor the window used while water vapor was being supplied to the lamp,but on the control window, the central Φ8 mm region through which thevacuum ultraviolet light was transmitted exhibited substance adhering ina film like manner. When plastic forceps were used to scrape outersurface 11, it was possible to scrape away the adhering material, whichwas found to be a weakly bound film-like substance adhering to outersurface 11.

At this point, elemental analysis was performed on the adheringmaterial. The results of elemental analysis by EPMA were similar tothose explained for Example 1.

As has been detailed above, when the lamp was operated with a feed ofwater vapor, it was confirmed that the adhesion of carbon to lighttransmitting window was prevented or suppressed.

This countermeasure makes it possible to inhibit the decline in thelight transmission rate of the light transmitting window, to therebyreduce maintenance costs associated with window replacement, and reduceoperational downtime of the lamp due to maintenance.

EXAMPLE 3

FIG. 3 shows a diagram of a microwave excited hydrogen ultraviolet lampwhich will be used to explain the third example of the first preferredembodiment according to this invention. Further elaboration ofstructural and operational elements that are similar to those of Example1 will be omitted. The specifications of light transmitting window 8were the same as those explained for Example 1. Again, a photodiode 12was positioned to receive the light output of lamp emitted light 9 as ameans of monitoring the amount of light output from said lamp.

Atmospheric components were supplied to vacuum zone 14 using the methodspecified below and these were adjusted to a specific gas partialpressure.

Atmospheric components were supplied to vacuum zone 14 by means of atube open to the atmosphere which, after adjusting the aperture withvariable leak valve 19, allowed the atmospheric components to travel viatube 16 b, and through a seal mechanism (not shown) to be introducedinto vacuum zone 14 via tube 16 a. The amount supplied was approximately1 sccm. Vacuum zone 14 was evacuated by means of a turbo molecular pump(evacuation rate 50 L/min, model TP-50 made by Mitsubishi HeavyIndustries, Ltd., not shown in figure), and dry pump (not shown)downstream. In this case, the atmospheric components were balancedinside the vacuum zone at 1 mtorr (1 millitorr). Accordingly, conditionswere such in vacuum zone 14 that the atmospheric components partialpressure was at least on the order of 1 mtorr, (with 0.2 mtorr of oxygenalone).

A valve aperture adjustment was also made to generate a partial pressureof 0.1 mtorr (with 0.02 mtorr of oxygen alone), but as will be describedlater, the effectiveness in carbon removal was similar.

Next, photodiode 12 was used to measure the changes over time of lightoutput during the operation of the microwave excited hydrogen lamp withthe above described structure.

First, hydrogen atoms were excited by discharge plasma 7, and vacuumultraviolet light was generated for 90 hours (about 4 days). Next, as acontrol, the atmospheric components were not supplied during operations,and the foregoing turbo molecular pump was used to create an environmentof 0.001 mtorr, and then the results of the two tests were compared.

When the lamp was operated with the supply of atmospheric components, nodiminishment of light transmission through light transmitting window 8due to carbon buildup could be observed. However, in the control test,carbon buildup caused the light transmission through light transmittingwindow 8 to decline from an initial value of 100% to a transmission rateof 35%.

The carbon 15 shown in FIG. 3 reflects the film-like adhesion of carbonthat was observed in the control, but when atmospheric components weresupplied during lamp operation, there was none of the adhesion of carbon15 on light transmitting window 8 that is shown in FIG. 3.

When outer surface 11 of light transmitting window 8 was observed usingan optical microscope after it had been used, no adherents were observedfor the window used while atmospheric components were being supplied tothe lamp, but on the control window, the central Φ8 mm region throughwhich the vacuum ultraviolet light was transmitted exhibited substanceadhering in a film like manner. When plastic forceps were used to scrapeouter surface 11, it was possible to scrape away the adhering material,which was found to be a weakly bound film-like substance adhering toouter surface 11.

At this point, elemental analysis was performed on the adheringmaterial. The results of elemental analysis by EPMA were similar tothose explained for Example 1.

As has been detailed above, when the lamp was operated with a feed ofatmospheric components, it was confirmed that the adhesion of carbon tolight transmitting window was prevented or suppressed.

This countermeasure makes it possible to inhibit the decline in thelight transmission rate of the light transmitting window, to therebyreduce maintenance costs associated with window replacement, andoperational downtime of the lamp due to maintenance.

EXAMPLE 4

FIG. 4 shows the structure of a microwave excited hydrogen ultravioletlamp used to explain the fourth example of the first preferredembodiment according to the present invention where the method forremoving carbon adhering to optical systems located in vacuum zone 14using output light will be described. Further elaboration of structuraland operational elements that are similar to those of Example 1 will beomitted. The specifications of light transmitting window 8 were the sameas those explained for Example 1.

Optical element 27 in FIG. 4 is positioned to be irradiated by lampemitted light 9. Carbon 15 was already adhering to both sides of opticalelement 27, and was produced by optical element 27's irradiation byvacuum ultraviolet light 9 while organic gases were present withinvacuum zone 14. Since carbon 15 was adhering, the transmission rate ofoptical element 27 had degraded and maintenance was required. The reasonwhy optical 27 had reached this state was because the lamp had beenoperated in a vacuum state.

An interference filter for vacuum ultraviolet light will be used here asan example of an optical element 27, used to describe the carbonremoval. The interference filter for vacuum ultraviolet light consistedof a MgF₂ substrate with a coating of a multi-layered optical film onits surface. This is a conventional structure for optical parts such asthis interference filter. This interference filter functions as a bandpass filter since it allows only light of a specific wavelength band topass through, but when carbon 15 adheres to its surface, itstransmission rate as an interference filter declines, and its functionas an optical element is thereby degraded. Accordingly, at a certainstage of diminished transmission rate, it is necessary to either removethe carbon or to replace the interference filter. In general, because ofthe delicate nature of optical filters such as interference filtershaving optical film coatings, cleaning them is very difficult. Thecleaning might change the properties of the optical film, and it is easyto introduce such defects as scratches during cleaning. Thus, there isessentially no effective cleaning method available, and one must electto replace the part. However, in general, interference filters areexpensive parts and cost becomes a problem. In Example 4, to prove theeffectiveness of the invention, the lamp was operated without a gassupply until the transmission rate of the interference level haddeclined to 50% (from an original value of 100%) to intentionallydegrade the transmission rate by half, and then the optical element 27was positioned within vacuum zone 14.

The method of Example 1 was used to supply oxygen gas to vacuum zone 14under conditions where the partial pressure of the oxygen gas in vacuumzone 14 was maintained at 1 mtorr. Experiments were also conducted byadjusting the valve aperture to deliver 10 mtorr, 5 mtorr, 2 mtorr, 0.1mtorr, and 0.05 mtorr, but as will be explained below, similar effectsin carbon removal were obtained.

Next, discharge plasma was used to excite the hydrogen atoms to causevacuum ultraviolet light to be emitted for 90 hours (about 4 days). Whenoxygen gas was supplied during lamp operations, adhering carbon 15 wasremoved from the surface of optical element 27, and the transmissionrate of optical element 27 was restored to virtually its original state.When the surface of optical element was observed under an opticalmicroscope, no adherents were noted.

As explained above, it was possible to clean off the carbon adhering tooptical element 27 by operating under a feed of oxygen gas.

This method makes it possible to inhibit the decline in the lighttransmission rate of the optical element, to thereby reduce maintenancecosts associated with optical element replacement, and operationaldowntime of the lamp due to maintenance.

EXAMPLE 5

FIG. 5 shows the microwave excited hydrogen ultraviolet lamp structureused to explain the fifth example of the first preferred embodimentaccording to this invention, wherein the removal of carbon adhering toan optical element located in vacuum zone 14 by using lamp lightemissions will be described. Further elaboration of structural andoperational elements that are similar to those of Example 1 will beomitted. The specifications of light transmitting window 8 were the sameas those explained for Example 1.

In FIG. 5, optical element 27 is positioned to receive the light emittedby lamp 9. Further explanation of the optical element will be omittedsince it is similar to that used in Example 4.

In Example 5, to verify the effects the effectiveness of the invention,the lamp was operated without a gas supply until the transmission rateof the interference level had declined 50% (from an original value of100%) to intentionally degrade the transmission rate by half, and thenthe optical element 27 was positioned within vacuum zone 14.

The method of Example 2 was used to supply water vapor to vacuum zone 14under conditions where the partial pressure of the water vapor in vacuumzone 14 was maintained at 1 mtorr. Experiments were also conducted byadjusting the valve aperture to deliver 5 mtorr, 2 mtorr, 0.01 mtorr,and 0.005 mtorr of water vapor partial pressure, but as will beexplained below, similar effects in carbon removal were obtained.

Next, discharge plasma was used to excite the hydrogen atoms to causevacuum ultraviolet light to be emitted for 90 hours (about 4 days). Whenwater vapor was supplied during lamp operations, adhering carbon 15 wasremoved from the surface of optical element 27, and the transmissionrate of optical element 27 was restored to virtually its original state.When the surface of optical element was observed under an opticalmicroscope, no adherents were noted.

As explained above, by operating under a feed of water vapor, it waspossible to clean off carbon 15 adhering to optical element 27.

This method made possible the restoration of the degraded opticalelement to thereby reduce maintenance costs associated with thereplacement of the optical element, as well as reduce lamp down time dueto maintenance.

EXAMPLE 6

FIG. 6 shows the microwave excited hydrogen ultraviolet lamp structureused to explain the sixth example of the first preferred embodimentaccording to this invention, wherein the removal of carbon 15 adheringto an optical element 27 located in vacuum zone 14 by using lamp lightemissions will be described. Further elaboration of structural andoperational elements that are similar to those of Example 1 will beomitted. The specifications of light transmitting window 8 were the sameas those explained for Example 1.

In FIG. 6, optical element 27 is positioned to receive the light emittedby lamp 9. Further explanation of the optical element will be omittedsince it is similar to that used in Example 4.

In Example 6, to verify the effects the effectiveness of the invention,the lamp was operated without a gas supply until the transmission rateof the interference level had declined 50% (from an original value of100%) to intentionally degrade the transmission rate by half, and thenthe optical element 27 was positioned within vacuum zone 14.

The method of Example 3 was used to supply atmospheric components tovacuum zone 14 under conditions where the partial pressure of theatmospheric components in vacuum zone 14 was maintained at 1 mtorr.

Experiments were also conducted by adjusting the valve aperture todeliver 2 mtorr, and 0.1 mtorr of atmospheric components' partialpressure, but as will be explained below, similar effects in carbonremoval were obtained.

Next, discharge plasma was used to excite the hydrogen atoms to causevacuum ultraviolet light to be emitted for 90 hours (about 4 days). Whenatmospheric components were supplied during lamp operations, adheringcarbon 15 was removed from the surface of optical element 27, and thetransmission rate of optical element 27 was restored to virtually itsoriginal state. When the surface of optical element was observed underan optical microscope, no adherents were noted.

As explained above, by operating under a feed of atmospheric components,it was possible to clean off carbon 15 adhering to optical element 27.This method made possible the restoration of the degraded opticalelement to thereby reduce maintenance costs associated with thereplacement of the optical element, and lamp down time due tomaintenance.

Second Preferred Embodiment

The second preferred embodiment of this invention will be explainedbelow as well with reference, in which a protective film is coated onthe light transmitting window for the purpose of preventing orsuppressing the degradation of the window. This invention is, however,not limited to this configuration, but it can naturally apply to thelamps that emit luminescent generated by electric discharging orheating, and laser devices if applicable.

FIG. 8 shows a diagram of a microwave-excited hydrogen ultraviolet lamp;it will be used to explain embodiments 1-3 of this invention. Flange 17for light transmitting window 8 is disc-shaped and its center alignswith the bore line of discharge tube 1 and contains an opening with adiameter that is larger than that of the discharge tube. Window flange17 contains an O-ring groove 13 b as a means to seal light transmittingwindow 8 over the foregoing opening, and a hollow, lid-shaped jig 20that includes bolt holes for attachment and an O-ring groove 13 a isemployed to attach discharge tube 1 and it further allows flange 17 tomaintain a vacuum.

The inside surface structure of jig 20 consists of two-step concentrichollow cylinders that envelop the space that houses light transmittingwindow 8 and discharge tube 1. On the end surface of the side thatencases discharge tube 1 is O-ring 13 c, which is installed in adiagonally cut surface that corresponds to the ring diameter. Further,threads (not shown) are cut on the outside circumferential surface ofthis end to allow installation a cylindrical, open ended cap 21, whichholds O-ring 13 c in place and defines the vacuum boundaries fordischarge tube 1. The window flange 17, jig 20 and cap 21 are all madefrom metal, in general stainless steel or aluminum, which are not goodsources of contamination, would be used, but the material is notconfined to these metals.

Now the operation of the microwave-excited hydrogen ultraviolet lampwith the above described structure will be explained. First, a 1/100dilution of hydrogen in helium gas is supplied through discharge gassupply opening 2 to discharge tube 1 at the rate of 20 sccm. Thedischarge gas is exhausted by means of a vacuum pump (not shown) throughdischarge gas exhaust opening 3, and the adjustment of the aperture of avalve (not show) that lies between discharge gas exhaust opening 3 andthe vacuum pump, controls the exhaust conductance to maintain the insideof discharge tube 1 at about 5 torr (665 Pa). The reason for flowing thedischarge gas from the side of light transmitting window 8 towarddischarge tube 1 is to make every effort to exhaust any materialgenerated inside discharge tube 1 by discharge plasma 7 in the directionaway from light transmitting window 8 so as to reduce sources ofcontamination to said window 8.

Microwave oscillator tuner 18 is cylindrical in shape and it is astructural element of the microwave oscillator that allows theadjustment of the microwave electromagnetic field distribution insidethe microwave oscillator; its inside diameter is the encasement ofdischarge tube 1. Further its structure is such that it can be insertedwhile aligned in the axial direction from the end surface of microwaveoscillator 4 and it can slide in the axial direction while maintainingelectrical conductivity with microwave oscillator 4. Tuner 18 is formedfrom copper or brass, the same material used for microwave oscillator 4.The function of said tuner 18 to adjust the microwave electromagneticfield distribution, which generates plasma 7 based upon the depth towhich it is inserted so as to concentrate the generation of microwavesin the center 6.

Next, 2.45 GHz, 50 W microwaves are supplied from microwave supplyconnector 5 to microwave oscillator 4. The supply of the microwaves maybe either continuous or intermittent. A regulator (not show)incorporated midway in the electrical power transmission line thatconnects the microwave power source with microwave oscillator. It can beadjusted to control the microwave power between the power source and theload (discharge plasma) to generate discharge plasma 7 inside ofdischarge tube 1. Hydrogen atoms excited by discharge plasma 7 generatevacuum ultraviolet light beams at the 103 nm and 122 nm wavelengths;they pass through light transmitting window 8 and allow irradiated lamplight 9 to be delivered to the outside.

MgF₂ (magnesium fluoride) mono-crystal was used to fabricate lighttransmitting window 8 and its crystal axis (c axis) was aligned to beperpendicular to the surface of the light transmitting window.

A thin film coating of Al₂O₃ (alumina) had been previously applied asprotective film 10A to surface 10 of light transmitting window 8 beforeinstalling it in the position shown in FIG. 8. The coating was appliedusing the ion beam sputtering type of film-forming method.

The ion beam film forming method will now be explained. An Ar gasenvironment maintained at a pressure of 0.1 Pa was used as the filmforming gas, and a 3 inch φ sintered Al₂O₃ target (purity 4N) wasbombarded using an Ar ion acceleration voltage of 20 kV to sputter theAl₂O₃ from the target onto surface 10 of light transmitting window 8 tocreate the film. The film thickness control was performed using a quartzoscillator, by creating a calibration curve in advance that detailed therelationship between the amount of variation in the number of quartzcrystal oscillations and the thickness of the film. By so doing, thefilm was formed to the desired thickness by varying oscillation timecorrespondingly.

The coating method used to generate protective film 10A is not confinedto the above described ion beam sputtering film forming method. It ispossible to produce films of the desired composition by appropriateselection of the method and device. Other possible methods include gasphase methods such vapor deposition, ion plating, CVD, etc.

The appropriate film thickness range for protective film 10A isdetermined based upon surface coverage situation for the optical systemsand the transmission required for the 122 nm vacuum ultraviolet light.

FIG. 13 shows the changes in light transmission vs. the thickness of theprotective film when an Al₂O₃ was applied as a light transmitting windowcoating, as compared with the initial state where no coating waspresent. As shown in FIG. 13, the degree to which the transmission rateof the optical system was decreased over that of its initial state is afunction of the thickness of the protective film. It is best to use thethinnest film possible to hold down this initial degradation. On theother hand, in order for the protective film to be effective, it has tocover all of the surfaces of the optical system. In general, the thinfilm is not of uniform film structure at the initial stage of itsapplication, it forms island-like structures on the optical surfaces toleave part of the optical surfaces exposed, and an effective protectivefilm has yet to be achieved.

Observations of the surface with an AFM (atomic force microscope) afterprotective film formation revealed that it was necessary to cover thesubstrate to a film thickness of 2 nm or greater in order to form aflat, smooth thin film.

Further, regarding abundantly thick film thickness of 20 nm or more,created with the objective of effectively protecting the surface of theoptical systems, it was found that with protective films of SiO₂ orAl₂O₃, MgO, TiO₂, or ZrO₂, due to their high absorption of vacuumultraviolet light, the characteristics of the optical systems upon whichthey were used were substantially degraded, and that the degradation andheat caused by the absorption by the protective film itself could causeit to peel off or otherwise cause a problem with the surface of theoptical system. Because this absorption of vacuum ultraviolet results inthe inability of optical systems to function as anticipated, the upperlimit for film thickness is set at 20 nm or greater, preferably 12 nm orgreater, or even more preferably 10 nm or greater.

In the present examples, a protective film thickness of 6 nm wasemployed. At this thickness for the protective film the transmissionrate for 122 nm wavelength light was 50% of the 100% transmission rateassigned to the initial state where no protective film was used.

Further, photodiode 12 was positioned to receive the lamp lightemissions 9 as a means of monitoring the light output of said lamp.

Next, photodiode 12 was used to measure any changes in the amount oflight output for the microwave-excited hydrogen ultraviolet lamp withthe above-described structure.

First, the hydrogen atoms were excited by plasma 7 and light wasgenerated in the vacuum ultraviolet light wavelength range for 90 hours(about 4 days). Next, as a control, light transmitting window 8 wasreplaced with one having no protective film, and the test was repeatedand results compared.

The following evaluation method was employed. The initial transmissionrate of the light transmitting window was T₁ (in the case of the controlexperiment, T₀=T₁, and then after use, to wit, 90 hours later, thereduced transmission rate was T₂, and then the change in transmissionrate ΔT[%] was computed as:ΔT=(T ₁ −T ₂)/90  Equation (1)Also, the ratio of change was expressed as the degradation rate K[%/hr.] as defined in the following equation.K=100·ΔT/T ₀  Equation (2)

It was possible to quickly quantify and evaluate the degradation oflight transmitting window 8 by comparing the magnitude of thedegradation rate K. Naturally, the lower the value of K, the milder thedegradation of the light transmitting window, the longer its longevityand the less frequently it required replacement.

Results indicated that when a protective film (Al₂O₃) was used on lighttransmitting window 8, the degradation rate K was 0.04%/Hr. On the otherhand, the degradation rate K for the control was 0.46%/Hr., about11-times that of the coated window. Based upon this evaluation, we foundthat protective film 10A on light transmitting window 8 delivered anapproximate factor of 10 improvement in longevity compared with nocoating being used.

To clarify the effects of protective film 10A, the results of XPSsurface analysis will be explained for light transmitting window 8coated with Al₂O₃ as protective film 10A, both before and after its usein a lamp, and as a control, for the surface of a light transmittingwindow not having a protective film, both before and after its use in alamp.

FIG. 9 shows the analytical results for the control before use. Thehorizontal axis is the argon time, the amount being proportional to thesputtering depth. Sputtering time zero min. indicates the initial stateprior to sputtering, and it corresponds to the analysis of the crystalsurface. In general with XPS analysis, the information obtained for theinitial state reflects the substance's natural contamination, detectedas the adsorption component for carbon, oxygen or the like. However,since there was virtually none, it was omitted from the analytical data.The vertical axis expresses the ratio at which the various elements werefound by XPS.

FIG. 9 shows that prior to use, there was no fluorine loss for thecontrol window. Although trace amounts of oxygen were found on thesurface, none was found to exist internally within the crystal. Theoxygen in the contamination material naturally adsorbed onto thesurface, was due to the argon sputtering which drove it into thecrystal. Accordingly, with regard to the presence or absence of oxygenwithin the crystal, the amount of oxygen shown in FIG. 9 should beinterpreted as the slight amount that should be used as the basis forthe calibration of other analytical results.

FIG. 10 shows the analytical results for the control after it had beenused. FIG. 10 clearly indicates a fluorine loss from the surface of thecontrol sample. A significant presence of oxygen was also found at thesame depth in the crystal as the fluorine-deficient layer. Thus, in thecontrol sample after use, the surface layer showed both an F deficiencyand oxidation. This surface state was the primary cause in the reductionin the transmission rate for the 122 nm wavelength vacuum ultravioletlight.

Next, protective film 10A of Al₂O₃ was applied to an approximate 5 nmthickness on light transmitting window 8, and FIG. 11 shows theanalytical results prior to its being used. The explanation of the graphaxes and interpretations are the same as for FIG. 9 and furtherelaboration will be omitted. Al (aluminum), one of the protective filmcomponents, has been newly added to the plot. FIG. 11 shows that thefluorine and magnesium synchronous profiles extend from the surface tothe inside, and that the synchronous profiles of oxygen and aluminumextend from the surface layer to the inside. Thus, the reason why theXPS analysis detected signals from fluorine and magnesium from thesurface layer, despite the protective film coating, is that theresolution of the XPS analysis in the depth direction is several nm.Thus, even if the ideal boundary distribution is attempted to bemeasured, it will not show up in the profile as a step because the widthof the resolution is unavoidably broad in shape. Further, with respectto the protective film thickness of 5 nm, if sputtering is not performedfor about 20 minutes, the MgF₂ crystals of the substrate will remainexposed. This is due to the differences in sputtering efficiency betweenAl₂O and MgF₂. Focusing on this point, it was possible to understand theareas where the F and Mg, and the O and Al profiles were synchronous.

Finally, FIG. 12 shows the analytical results after use of lighttransmitting window 8 that had been coated with about a 6 nm thicknessof Al₂O₃ as the protective film. The explanation of the graph axes andinterpretations are the same as for FIG. 9, and further elaboration willbe omitted. FIG. 12 shows that the profiles of the oxygen and aluminumfrom the surface toward the inside were synchronous. Further, theexistence of oxygen inside of the crystal was not confirmed. This makesit clear that the invasion of oxygen into the interior of the crystalwas prevented by the protective film.

On the other hand, the profiles of the fluorine and magnesium from thesurface and into the center were not synchronous. It was clear thatfluorine had penetrated into the Al2O3 of protective film 10A. However,due to the presence of protective film 10A, although the fluorine waspresent inside of the protective film, the fluorine had not beenexpelled altogether to cause a fluorine deficiency, as was the case inthe control, which makes it easy to imagine the mechanism by whichoxygen invades as a replacement. In fact, the formation of a fluorinedeficiency layer and an oxide layer as describe using FIG. 10 is easilyexplained if the case for no protective film in FIG. 12 is considered.

As has been explained above, by using protective film 10A as a coatingon light transmitting window 8, it is possible to suppress thegeneration of a fluorine deficient layer, and prevent or suppress thepresence of oxygen (an oxide layer) inside the crystal, and further,when compared to the control, the light transmitting window having theprotective film coating delivered a degradation rate K that was lower byabout a factor of 10.

Also, when using a SiO₂ coating (film thickness 6 nm) as the protectivefilm for light transmitting window 8, the degradation rate K is0.06%/Hr. On the other hand, the degradation rate K of the control was0.46%/Hr. This confirms that a similar level of protection was achievedeven when using SiO₂ in the protective film, about an 8-fold improvementin the degradation rate K.

Further, just as with Al₂O₃, metal oxides such as MgO, TiO₂, ZrO₂, whichexhibit less discoloration under ultraviolet light irradiation thanfluorine compounds, may also be used as materials for the protectivefilms.

As has been described above, optical systems according to the presentinvention having protective films formed upon them, have opticalproperties themselves (e.g. if it is a light transmitting window, itwould be the light transmission rate) that are inferior to thoseprovided in the pre-coated state, without the protective film. However,it is not appropriate to evaluate those optical systems alone, it isimportant to evaluate them as parts incorporated into a light outputdevice overall as a part of a system employing a light output device. Towit, it is possible to compensate for the aforementioned initialinferiority of the optic systems in the optical output device, and matchthe light output to the specifications required for the system, whichmakes it possible to sustain the output of the light output device andincrease its longevity to thereby achieve the objective of providinglight output devices in which the frequency of maintenance and the costmaintenance for their light transmitting windows, etc. are substantiallyreduced.

Also, the use of this invention for light sources used in measurementapplications is especially beneficial. An example is performing longterm monitoring or the like of the generation of environmentalpollutants. Generally, when making this sort of measurement, the levelof the signal and sensitivity in measurement is proportional to thesquare of the light output. As described above, in the prior art, themeasurement sensitivity of the light source was improved by improvingthe output of the light source, but the resulting degradation of theoptical systems made it necessary to suppress the degradation of theoptical system that reduced the light output, and diminished thesensitivity of the measurement. The optical system used in the presentinvention lengthens the longevity of the light output device, andmaintains output properties that are more stable over the long term toresolve the foregoing problem and provide a light output device that isappropriate for use in long term environmental monitoring.

The implementation examples above used the example of the lighttransmitting window, but it may as well be applied to devices usinglight reflecting mirrors (windows). Examples of such light reflectingmirrors are the reflecting mirrors used in laser oscillators andfocusing mirrors used in lamps. Thus, the light reflecting mirrors couldbe used in similar implementation examples.

EFFECTS OF THE INVENTION

As specified above, the present invention makes possible the preventionor suppression of the degradation of optical systems specifically due tocarbon buildup that reduces the transmission rate and determines thelongevity of the foregoing systems and optical elements to therebyreduce the frequency of maintenance operations to replace opticalsystems and reduce operational costs in a variety of optical apparatusemploying high photon energy light such as conventional ultravioletlight or vacuum ultraviolet light when used in systems using opticalelements one or a combination of optical effects such as transmission,refraction, reflection, spectrum generation, interference, for examplewhen said transmitting or reflecting optical elements are positionedwithin the boundaries of a near vacuum zone where decomposable organiccomponents can cause degradation of optical elements along the lightpath in the vacuum zone for diffraction, refraction, spectrumgeneration, transmission, or analytical position adjusting opticalelements or other surfaces subjected to irradiation, includingcontainers, seal materials and position adjusting equipment for opticalelements are present, such as exposure apparatus (steppers) and colorplates that are used in the semiconductor industry with vacuumultraviolet light.

Specifically, by preventing or suppressing the decline in the lighttransmission rate in optical systems caused by the buildup of carbon ontheir surfaces, it is possible to prevent or suppress the degradation ofsaid optical systems and thereby reduce the frequency of maintenanceoperations to replace, etc., optical systems and lower operating costs.

Further, by preventing or suppressing the buildup of carbon onirradiated surfaces and emission surfaces in optical systems along thelight path in a vacuum zone, it is possible to extend the longevity ofdownstream equipment and improve the reliability of the equipment.

In particular, since the present invention makes possible the preventionor suppression of the diminishment of the optical transmission rate dueto carbon buildup on the light transmitting window and other opticalelements, the required maintenance interval for the cleaning orreplacement of the light transmitting window, etc., may be extended tothereby contribute to improving the operational rate of the equipmentand reducing maintenance costs.

Also, by preventing or suppressing the buildup of carbon on irradiatedsurfaces and emission surfaces of optical elements and optical systemsused in vacuum zones in which light output apparatus irradiate light, itis possible to extend the longevity of downstream apparatus and improvethe reliability of the equipment. Further, the method of the presentinvention can be employed to irradiate optical elements that have beenpreviously degraded by carbon buildup to irradiate these degradedoptical elements and restore them to their original condition.

Thus, through the use of this invention, the maintenance cycle for thecleaning or replacement of optical elements used with inside theaforementioned vacuum zones can be lengthened, to thereby contribute tothe improvement in the operational rate of the equipment and thereduction of maintenance costs.

Since the present invention, as described above, makes it possible toprevent or suppress the deterioration of optical systems and extend themaintenance cycle at which they must be replaced, it contributes to theimprovement of the operational rate of the equipment and to thereduction of maintenance costs.

Furthermore, by incorporating optical systems according to the presentinvention into equipment that utilizes light, it is possible to extendthe longevity of such equipment and secure stable output characteristicsfrom that equipment over the long term.

1. A method to use an optical apparatus, comprising steps of: applyingin advance a protective film of a metal oxide selected from the groupconsisting of Al₂O₃, MgO, TiO₂, and ZrO₂ to an optical system, wherein atotal thickness of all film applied to the optical system is 2 nm-10 nm,and said protective film suppresses the stripping off of a structuralelement from the surface of a base stock of said optical system or theoxidation of the surface of the base stock of said optical system, bythe irradiation of vacuum ultraviolet light over time or the plasmaexposure to the base stock, incorporating said optical system into adesired device that has vacuum ultraviolet light sources or plasma lightsources which has higher photon energy than an absorption wavelength ofthe base stock of said optical system, and forming the protective filmon the optical system, wherein the optical system comprises ofmono-crystal fluoride material having the crystal axis (the c axis)along the direction of the light irradiation, and a perpendicularsurface of said protective film is coated by Si02 or metal oxides. 2.The method to use an optical apparatus according to claim 1, whereinsaid light sources provide adequate light output to compensate for theinitial degradation of said optical system due to said protective film,and said optical system is provided on the optical path of said lightsource for suppressing the degradation of optical properties after saidinitial degradation of said apparatus.
 3. The method to use an opticalapparatus according to claim 1, wherein the desired device comprises alight transmitting window provided in an environment that is exposed byvacuum ultraviolet light at a wavelength of 122 nm from amicrowave-excited hydrogen ultraviolet lamp, MgF₂ is used as the lighttransmitting window, the protective film of a metal oxide selected fromthe group consisting of AI₂0₃, MgO, TiO₂, and ZrO₂ is applied to thelight transmitting window in advance with a thickness of 2 nm-10 nm; andsaid optical system is incorporated into said desired device that hasvacuum ultraviolet light sources at a wavelength of 122 nm or plasmalight sources.
 4. The method to use an optical apparatus according toclaim 3, wherein said vacuum ultraviolet light sources at a wavelengthof 122 nm provide adequate light output to compensate for the initialdegradation of said light transmitting window due to said protectivefilm, and said light transmitting window is provided on the optical pathof said light source for suppressing the degradation of opticalproperties after said initial degradation of said apparatus and the filmthickness of the protective film which is coated on said lighttransmitting window is formed no less than 2 nm nor more than 10 nm inorder to maintain 30 to 40% of the optical properties of the base stock.